It has previously been proposed to utilize current supply sources in which current flow to a load from a pair of power terminals is controlled by a transistor, which may be in form of a composite unit such as a Darlington-connected transistor unit. To provide appropriate control current for the transistor unit, a current supply therefor may, in turn, use a transistor circuit which is connected to the same supply terminals as the control transistor unit and load. The voltage at the supply terminals may, however, vary and circuitry has been developed to provide base current to the control transistor independently of voltage variations of the supply source.
Circuits of this type, in which an output transistor is used to connect or disconnect a load, can be applied for loads which are merely resistive, such as incandescent lamps, heaters, or the like, or which may be inductive, such as relays. Circuit arrangements can be constructed in discrete technology, in monolithically integrated technology, or in hybrid technology. The danger arises for all such transistor circuits, however, that if a short circuit should occur, for example due to failure in the load or the like, the current flow through the output transistor will be such that damage to, or destruction of, the output transistor may result.
Reference power sources, for example to supply reference or control power to the output transistor to control load current, referred to above, have previously been proposed in which the reference current becomes independent of supply line voltage. The above-referenced publication, Philips Technische Rundschau, Vol. 32, Jahrgang, 1971/72, Nr. 1, Page 8, FIG. 10 (Philips Technical Review), describes a reference source which furnishes reference supply current essentially independent of supply voltage. The current source is constructed as a ring network, using differential amplifier technology. Another type of ring-current supply source is described in the referenced German patent disclosure DE-OS No. 31 46 600, which uses a current mirror circuit and, additionally, has a temperature coefficient for the current which is readily reproducible. The circuit utilizes, for example, npn transistors, from which theoretically correct currents can be obtained; alternatively, ring current sources can be used, for example a current source having two separate source circuits, in which one of the source circuits is the reference current for the other. The final output current, then, is controlled by one of the two current sources. This current source is particularly suitable for integrated circuits, has low internal resistance, and is independent of supply voltage. The combination of two reference current sources, including a resistor which controls the output current, can be so interconnected that the two current sources generate similar currents in two branches.